It has been known to provide stereoscopic viewing with using parallax by generating a stereoscopic image by combining more than one images of the same subject taken from different positions, and stereoscopically displaying the stereoscopic image. As a specific example of the stereoscopic display, parallel viewing with naked eyes is known, where the stereoscopic display is achieved by arranging the more than one images side by side. Further, the stereoscopic display may be achieved by generating a stereoscopic image by combining the more than one images, for example, by overlapping the images with changing the colors of the images into, for example, red and blue, or by overlapping the images with providing different polarization directions of the images. In these cases, the stereoscopic viewing is effected by using image separating glasses, such as red-and-blue glasses or polarization glasses, to provide a merged view of the stereoscopically displayed stereoscopic image (anaglyph system, polarization filter system).
Furthermore, the stereoscopic viewing may be achieved by displaying the more than one images on a stereoscopic display monitor which allows the stereoscopic viewing of the images, such as in a parallax barrier system or a lenticular system, without using the polarization glasses, etc. In this case, the stereoscopic display is achieved by cutting the more than one images into vertical strips and alternately arranging the strips to generate a stereoscopic image. Moreover, a method for providing the stereoscopic display using afterimage effect has been proposed, in which left and right images are alternately displayed by being switched at a high speed with changing the direction of light beams from the left and right images through the use of the image separation glasses or attaching an optical element on a liquid crystal display (scanning backlight system).
When the stereoscopic viewing is provided in such manners, a preferred level of stereoscopic effect varies among users who view the stereoscopic images. In order to address this problem, a technique to generate a stereoscopic image has been proposed, where an instruction to control a parallax level of the stereoscopic image is received while the image is stereoscopically displayed, and the stereoscopic image is generated according to the instructed parallax level (see Japanese Unexamined Patent Publication No. 2004-129186, which is hereinafter referred to as Patent Document 1). Another technique to generate a stereoscopic image has been proposed, where the stereoscopic effect is controlled while the stereoscopic image being displayed is stereoscopically viewed (see Japanese Unexamined Patent Publication No. 10 (1998)-090814, which is hereinafter referred to as Patent Document 2). A technique to control the stereoscopic effect during a video game has also been proposed (see Japanese Unexamined Patent Publication No. 9(1997)-192349, which is hereinafter referred to as Patent Document 3).
In the techniques disclosed in Patent Documents 1 to 3, however, the stereoscopic image generated from more than one images is stored. therefore, after the stereoscopic image has been generated, only the stereoscopic image with the stereoscopic effect that was fixed when the image was generated can be displayed, and the stereoscopic effect of the image cannot be changed even when the user wishes to change the stereoscopic effect.